In recent years, digital imaging apparatuses (flat panel detectors (FPDs)) which convert X-rays to electrical signals using an X-ray detector including a solid-state image sensor have become widely used to replace conventional film photographing apparatuses. Digital imaging apparatuses allow X-ray images to be easily handled as digital information. Thus, in terms of image processing, information sharing, management, and cost, for example, digital imaging apparatuses have great advantages over conventional film imaging apparatuses.
However, images captured by such imaging apparatuses contain offset noise. The offset noise is generated by a photoelectric conversion circuit and a read circuit. The offset noise is caused due to several factors, for example, a dark current in photoelectric conversion elements, a leak current in switching elements, and an offset voltage in amplifiers in the read circuit. Such offset noise is gradually accumulated with time when the imaging apparatus is kept in an imaging wait state.
U.S. Pat. No. 7,593,508 discusses a technique for reducing offset noise contained in an image by periodically performing initialization driving in which offset noise accumulated before imaging is read from an X-ray detector. U.S. Pat. No. 7,593,508 also discusses a technique in which X-ray imaging is performed after initialization driving is performed once again in response to an imaging instruction.
FIG. 8 is a timing chart illustrating a method for driving a exemplary X-ray imaging apparatus. Timing TC101 indicates an imaging instruction signal input to the X-ray imaging apparatus from an external unit. Timing TC102 indicates an imaging enabling signal output from the X-ray imaging apparatus. Timing TC103 indicates the timing of X-ray irradiation which is conducted in response to the imaging enabling signal. Timing TC104 indicates a driving state in an X-ray detector. Timing TC105 indicates the timing of acquisition of images from the X-ray detector. Timing TC106 indicates the timing of acquisition of an offset-corrected image.
As indicated by the timing TC104, during an imaging wait state, the X-ray detector repeatedly performs initialization driving R at fixed time intervals T1. In this state, upon receipt of the imaging instruction signal after the completion of initialization driving R, the X-ray detector performs pre-imaging initialization driving R. Then, the X-ray irradiation enabling signal is output to cause X-ray irradiation. After the X-ray irradiation is complete, the X-ray detector starts read driving D to read image signals.
On the condition that the image sensor repeatedly performs initialization which includes output of the offset noise data at certain intervals, if the imaging instruction signal is input within a sufficiently short period of time after the completion of the initialization, initialization does not need be performed again after the instruction signal is input.